DISCUSSION OF FINAL RESULTS. 23 



It may be noted that the difference between the new value and the 

 old value of Stas is somewhat less than in the case of sodium. Stas's 

 results with potassic chloride led him to the value 39.146, about 0.032 

 hie-her than the new result, while with sodic chloride the difference was 



o 



0.042. A large part of this difference in each case is due to Stas's error 

 in the atomic weight of chlorine ; corrected for this error, the differences 

 become 0.016 and 0.026, respectively. The differences are to be referred 

 to the same causes in this case as in the others - - namely, to Stas's incom- 

 plete knowledge concerning solutions of argentic chloride, to his practice 

 of dropping solid salt into the precipitating solution, and to the presence 

 in his preparations of traces of impurity taken from containing vessels. 

 In the case of potassium one or more of these errors must have been 

 less than in the case of sodium. 



The results of Richards and Archibald, although very few in number 

 and not intended to figure in a discussion of this kind, were somewhat 

 better -- probably because solid salt was never used directly in precipita- 

 tion. Assuming the present research to yield the true value, their errors 

 in the two series were, respectively, 0.009 and 0.014. The result of 

 Archibald alone for the ratio of silver to potassium chloride was about 

 the same amount (0.008) different from the present value; but his result 

 for the other ratio was less satisfactory, having an error of 0.021, even 

 greater than Stas's. These differences are not surprising, because less 

 was known at that time than at present concerning the behavior of 

 argentic chloride in solution. 



Although the present paper presents strong evidence that the atomic 

 weight of potassium is really as low as 39.114, more remains to be done. 

 It would have been desirable to have used also some other wholly different 

 method of preparing potassic chloride, and, moreover, to have evaporated 

 large samples of the salt in nitrogen in order to discover a possible non- 

 volatile residue. It is very doubtful if these additional experiments would 

 have altered the present result, especially considering the precautions 

 taken in the work and the result of the following research ; but never- 

 theless it is planned to pursue these matters further. 



Even when every conceivable precaution is taken, a single salt is not 

 an adequate basis for the certain decision of an atomic weight. For 

 this reason a parallel investigation on potassic bromide was simultaneously 

 in progress at the Chemical Laboratory of Harvard College. The next 

 communication, describing this other research, must be considered in 

 connection with the work which has just been described. As will be seen, 

 excellent confirmation of the present work is afforded by the work with 

 the bromide. 



